项目中的文件无法运行，无法交换。 它们之间也有问题-/重复符号/

Question

我在Xcode中用C ++编写代码并遇到了这个问题：

 duplicate symbol '_main' in:
    /Users/admin/Library/Developer/Xcode/DerivedData/stepic-czeoumulvkebkydlwswhywbxwqvw/Build/Intermediates.noindex/stepic.build/Debug/stepic.build/Objects-normal/x86_64/4.main.o
    /Users/admin/Library/Developer/Xcode/DerivedData/stepic-czeoumulvkebkydlwswhywbxwqvw/Build/Intermediates.noindex/stepic.build/Debug/stepic.build/Objects-normal/x86_64/vector.main.o
duplicate symbol '_main' in:
    /Users/admin/Library/Developer/Xcode/DerivedData/stepic-czeoumulvkebkydlwswhywbxwqvw/Build/Intermediates.noindex/stepic.build/Debug/stepic.build/Objects-normal/x86_64/4.main.o
    /Users/admin/Library/Developer/Xcode/DerivedData/stepic-czeoumulvkebkydlwswhywbxwqvw/Build/Intermediates.noindex/stepic.build/Debug/stepic.build/Objects-normal/x86_64/theory.main.o
duplicate symbol '_main' in:
    /Users/admin/Library/Developer/Xcode/DerivedData/stepic-czeoumulvkebkydlwswhywbxwqvw/Build/Intermediates.noindex/stepic.build/Debug/stepic.build/Objects-normal/x86_64/4.main.o
    /Users/admin/Library/Developer/Xcode/DerivedData/stepic-czeoumulvkebkydlwswhywbxwqvw/Build/Intermediates.noindex/stepic.build/Debug/stepic.build/Objects-normal/x86_64/7taskmain.o
ld: 3 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

有人可以帮忙吗？ 为什么它从不同的文件写入重复的/ main /？